1. Field of the Invention
The present invention relates to limit sensing devices and more particularly to non-resettable, overtemperature sensing devices for indicating when a predetermined temperature limit has been reached in a fluid conduit. The sensing device is particularly designed for applications on aircraft engines including jet engines where it is desirable and indeed, critical in some instances, to determine whether or not a particular fluid line or engine part has exceeded a specific operating temperature.
Moreover, it is desirable that the limit sensing indicator be tamper-proof so that it cannot be accidentally or intentionally reset and thereby will always provide a positive indication of whether or not a particular limit has been reached or exceeded. The limit sensing indicator of the present invention has to be rugged to be useful in a harsh environment such as that encountered on a jet aircraft engine, or the like and in addition, must be highly reliable to assure that no false overtemperature indications are given as well as insuring that a true indication is provided when a critical overtemperature condition does, in fact, occur.
2. Brief Description of the Prior Art
Over the years, a variety of different chemical and/or mechanical systems have been provided for sensing and indicating the occurrence of a temperature limit being reached. However, none have been particularly well suited for the rough and harsh environmental conditions that obtain in jet engine applications and the like, wherein high temperatures as well as extremely low temperatures are alternately encountered in relatively short spans of time. In addition to temperature changes, rapid changes in pressure, physical orientation and G-forces are normally encountered and these conditions must not affect the operational characteristics and reliability of the temperature limit sensing function.
Gillespie Patent No. 2,508,510 is directed towards an attachment for railroad car journal boxes which provide a system for stopping a train when a wheel axle journal becomes heated to a predetermined degree. Vogt Patent No. 3,452,706 is directed towards a temperature indicating device for providing a visual indication of a specific or abnormal temperature condition. U.S. Pat. No. 3,062,989 discloses a capacitor protective circuit and apparatus for protecting a voltage regulator capacitor in a distribution transformer from excessively high voltage build-ups.
German Patent No. 64941 discloses a hot box indicator wherein a washer of fusible metal is used to retain a stud-like plunger against the force of a spring. Barber, U.S. Pat. No. 2,952,238 discloses a hot bearing signal device wherein metallic bonding material such as solder is used to retain the flange of an indicator in a retracted position against the action of a coiled spring. Muncheryan U.S. Pat. No. 2,953,921 discloses a temperature indicating device and closure cap utilizing a bi-metal part to provide the impetus for moving a colored indicator in response to temperature changes. Munroe U.S. Pat. No. 3,401,666 discloses a temperature detecting actuator for bearings employing a sleeve bonded to a body member by a thin film of fusible material. Kliewer U.S. Pat. Nos. 3,559,615 and 3,626,897 disclose temperature signalling devices employing a slug of fusible material for restraining an indicator against torsional loading. Reicher et al U.S. Pat. No. 4,220,300 discloses a thermal monitor for railroad wheels employing a lead washer which melts when overheated forming an open cavity, and Busch et al U.S. Pat. No. 4,818,119 discloses a hot bearing warning bolt having a fusible element mounted in the head of the bolt.